Gas diffusion electrode for carbon dioxide utilization, method for producing same, and electrolytic cell having a gas diffusion electrode

ABSTRACT

A gas diffusion electrode for carbon dioxide utilization, including a metal substrate and an electrically conductive catalyst layer, which is applied to the metal substrate and has hydrophilic pores and/or channels and hydrophobic pores and/or channels, wherein the catalyst layer includes metal particles and a first polymeric binding material; and a porous gas diffusion layer containing the first polymeric binding material is formed on the surface of the catalyst layer. A method produces a gas diffusion electrode for CO2 utilization and an electrolytic cell has a corresponding gas diffusion electrode.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2019/064572 filed 5 Jun. 2019, and claims the benefit thereof. The International Application claims the benefit of German Application No. DE 10 2018 210 457.3 filed 27 Jun. 2018. All of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The present invention relates to a gas diffusion electrode for utilization of carbon dioxide and also a process for producing a gas diffusion electrode. The invention further relates to an electrolysis system having a corresponding gas diffusion electrode.

BACKGROUND OF INVENTION

At present, about 80% of worldwide energy consumption is covered by the combustion of fossil fuels. About 34000 million metric tons of the greenhouse gas carbon dioxide (CO₂) are emitted into the atmosphere via these combustion processes every year worldwide. The major part of carbon dioxide is disposed of via this liberation into the atmosphere (in the case of large brown coal power stations more than 50000 t per day).

Owing to the increasing scarcity of fossil fuel resources and the volatile availability of renewable energy sources, research into the reduction of CO₂ is becoming of ever greater interest. Here, CO₂ emissions are decreased and the CO₂ could be utilized as inexpensive carbon source.

The discussion regarding the adverse effects of CO₂ on the climate has led to reutilization of CO₂ being considered. However, CO₂ is thermodynamically in a very low position and can therefore be reduced again to give usable products only with difficulty.

A natural degradation of carbon dioxide occurs, for example, by means of photosynthesis. Here, carbon dioxide is converted into carbohydrates in a process divided into many substeps over time and spatially on a molecular level. However, this process cannot readily be carried over to an industrial scale. A copy of the natural photosynthesis process using industrial photocatalysis has hitherto not been sufficiently efficient.

A further method is the electrochemical reduction of carbon dioxide. Systematic studies on the electrochemical reduction of carbon dioxide are still a relatively young field of development. Only since a few years ago have efforts been made to develop an electrochemical system which can reduce an acceptable amount of carbon dioxide.

Electrolysis systems having gas diffusion electrodes have now been used to an increased extent for this purpose. These systems usually consist of a cathode space and an anode space. To achieve an effective conversion of the CO₂ used, the cathode is ideally configured as porous gas diffusion electrode. Gas diffusion electrodes (GDE) are porous electrodes in which liquid, solid and gaseous phases are present and the electrically conductive catalyst catalyzes the electrochemical reaction between the liquid phase and the gaseous phase.

A typical problem occurring in the case of gas diffusion electrodes used for CO₂ reduction which are in contact with an electrolyte (for example KHCO₃, K₂SO₄, KOH or mixtures thereof) is that of avoiding undesirable secondary reactions in the electrolyte-side region of the gas diffusion electrode. Here, the gas diffusion electrode has to ensure sufficient supply of CO₂ and the respective electrolyte to the catalytically active sites by means of gas and ion transport.

Gas diffusion electrodes are frequently produced by means of a roller calendering process as is also disclosed in U.S. Pat. No. 2,013,010 190 6 A1. Here, the catalytically active metallic particles used for forming the gas diffusion electrode are mixed with hydrophobic particles such as PTFE, the resulting mixture is applied to a metallic support and arranged between two PTFE films before being calendered. The shear force enables the PTFE to flow and a network of PTFE binds the catalytically active particles together. The porosity and to a certain extent also the pore size can be modified via the compression force and the particle size.

However, gas diffusion electrodes produced in this way tend to have the limiting pore diameter only within the gas diffusion electrode but not on the surface. The pore size diameter facing the electrolyte side is comparatively large compared to gas diffusion electrodes having a gas diffusion layer, so that undesirable flooding behavior of the pore system is observed. Furthermore, gas diffusion electrodes having large pore openings or a low hydrophobicity display strong electrolyte permeation through the respective gas diffusion electrode. The permeation is controlled by the electric field gradient, i.e. electroosmosis. This effect leads to an undesirable decrease in the Faraday efficiency of the gaseous products CO or ethylene.

SUMMARY OF INVENTION

It is therefore an object of the invention to provide a possibility for electrochemical utilization of CO₂ which is more efficient compared to the prior art.

This object is achieved according to the invention by the features of the independent claims. Advantageous embodiments of the invention are set forth in the dependent claims and the following description.

The gas diffusion electrode of the invention is used for the utilization of carbon dioxide and comprises a metallic support and an electrically conductive catalyst layer which has been applied to this metallic support and has hydrophilic pores and/or channels and hydrophobic pores and/or channels. The catalyst layer comprises metallic particles and a first polymeric binder material, wherein a porous gas diffusion layer containing the first polymeric binder material has been formed on the surface of the catalyst layer.

The polymeric gas diffusion layer formed on the surface (the side of the catalyst layer or the gas electrode facing the electrolyte in an electrolysis cell) of the catalyst layer represents the reaction zone outside the gas diffusion electrode which is in contact with the reactants, in particular CO₂ and the electrolyte.

The gas diffusion layer here sets the limiting pore diameter for the total gas diffusion electrode. Both the degree of hydrophobicity and the pore size of the gas diffusion layer can be controlled via targeted selection of the binder material used, the size of the metallic, catalytically active particles and the parameters in the production process for the gas diffusion electrode.

The gas diffusion layer advantageously has a porosity of more than 70%. The thickness of the gas diffusion layer is advantageously in the range from 150 μm to 500 μm.

The thickness of the catalyst layer is advantageously in the range from 5 nm to 500 nm. The differential pressure based on passage of a fluid medium through the catalyst layer and also the hydrostatic pressure based on passage of a fluid medium through the outer layer can be influenced or set via such a catalyst layer.

A fluoropolymer is advantageously used as first polymeric binder material. In particular, from 3% by weight to 15% by weight of the first polymeric binder material is used. The use of polyvinylidene fluoride (PVDF) is particularly advantageous here. This polymer allows formation of the desired outer layer in the production of the gas diffusion electrode.

In addition, the first polymeric binder material is advantageously embedded partly within the pores and/or channels of the catalyst layer. In this way, a hydrophobic “subnetwork” is additionally formed within the pores of the catalyst layer, which increases the hydrophobicity of the catalyst layer and thus the gas diffusion electrode overall.

The differential pressure based on passage of a fluid medium through the gas diffusion layer and also the hydrostatic pressure based on passage of a fluid medium through the gas diffusion layer can be influenced or set via such a gas diffusion layer. The differential pressure based on passage of a fluid medium through the gas diffusion layer is in the range from 20 mbar to 220 mbar, in particular in the range from 60 mbar to 200 mbar. The hydrostatic pressure based on passage of a fluid medium through the gas diffusion layer is advantageously in the range from 20 mbar to 1000 mbar and in particular in the range from 200 mbar to 1000 mbar.

The pore size of the catalyst layer is advantageously in the range from 0.3 μm to 5 μm. The pore size of the catalyst layer is particularly advantageously in the range from 2 μm to 3 μm. The particle size of the metallic particles is advantageously in the range from 500 nm to 5 μm and particularly advantageously in the range from 2 μm to 3 μm.

The metallic particles are advantageously precoated at least in subregions with a second polymeric binder material. This increases the hydrophobicity of the gas diffusion electrode further. As second polymeric binder material (binder polymer), advantage is given to using PTFE (polytetrafluoroethylene). As metallic particles, advantage is given to using silver particles. The use of copper particles or other catalytically active particles is also possible. The metallic particles used are advantageously coated at least in subregions with the first polymeric binder material.

The metallic support is advantageously configured as a metallic gauze (or a corresponding sheet-like structure made of wire). Here, the material of the support is advantageously matched to the metallic particles used. A silver gauze is advantageously used as metallic support.

The gas diffusion electrode is particularly advantageously made by means of an extraction process. This process makes growth of the desired thin gas diffusion layer on the surface of the catalyst layer of the gas diffusion electrode possible.

The process of the invention serves to produce a gas diffusion electrode for utilization of CO₂. The process comprises mixing of metallic particles with a first binder material to form a suspension, application of the suspension to a metallic support and introduction of the metallic support loaded with the suspension into a precipitation bath to form an electrically conductive catalyst layer. A porous gas diffusion layer containing the first polymeric binder material is formed on the surface of the catalyst layer within the precipitation bath.

The above-described process is an extraction process (“inversion casting” process, phase inversion). Manufacture of the gas diffusion electrode by means of this process makes it possible for a thin gas diffusion layer whose pore diameter is limiting for the total gas diffusion electrode to be produced on the surface of the catalyst layer. Here, the first binder material, the size of the metallic, catalytically active particles and the parameters in the production process for the gas diffusion electrode influence the degree of hydrophobicity and also the pore size of the gas diffusion layer.

Furthermore, an intensive connection between the metallic particles and the binder materials used is achieved by means of the extraction process, as a result of which the mechanical stability of the gas diffusion electrode is improved compared to conventional gas diffusion electrodes.

The production of “tailored” gas diffusion layers on the surface of the catalyst layer of gas diffusion electrodes is particularly advantageous because the Faraday efficiency is improved as a result of the reduced risk of flooding of the gas diffusion electrode and thus two-phase secondary reactions such as evolution of hydrogen are reduced.

Furthermore, the gas diffusion electrode can be made smaller since a greater differential pressure through the gas diffusion electrode is less sensitive to the hydrostatic pressure of the electrolyte.

In addition, the production of the gas diffusion electrode is associated with a smaller outlay since one process step, namely activation of the electrode (oxidation of additional metal oxides), can be omitted. The metallic particles can be used directly.

A further advantage is that the gas diffusion electrode is simpler to integrate into electrolysis systems because of the decreased passage of electrolyte compared to conventional electrodes.

A mixture of water and isopropanol is advantageously used as precipitation bath. This mixture represents a “nonsolvent” for the polymeric binder materials and as a result of diffusion brings about exchange of solvent and nonsolvent and thus phase separation.

The first polymeric binder material solidifies here and forms the gas diffusion layer on the surface of the catalyst layer.

The advantages and advantageous embodiments described for the gas diffusion electrode of the invention apply equally to the process of the invention and can accordingly be carried over analogously to this.

The electrolysis cell of the invention comprises a gas diffusion electrode as per one of the above-described embodiments. The gas diffusion electrode is advantageously used as cathode here. The electrolysis cell is advantageously configured on the cathode side for the reduction of carbon dioxide.

The further constituents of the electrolysis cell, for instance the anode, optionally one or more membranes, feed conduit(s) and discharge conduit(s), the voltage source and further optional facilities such as cooling or heating devices, are essentially variable for the purposes of the invention. The same applies to the anolytes and/or catholytes which are used in such an electrolysis cell.

Overall, the use of a gas diffusion electrode according to the invention in an appropriate electrolysis system or in an electrolysis cell leads to greater efficiency of the electrochemical system and thus makes the end product more competitive. This is additionally supported by the corresponding process according to the invention for producing the gas diffusion electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

Working examples of the invention are explained in more detail below with the aid of a drawing. The drawing shows:

FIG. 1 a schematic depiction of a section of a gas diffusion electrode made by means of an extraction process,

FIG. 2 a schematic depiction of a section of a gas diffusion electrode made by means of a calendering process,

FIG. 3 a section of the gas diffusion electrode of FIG. 2, and

FIG. 4 a further section of the gas diffusion electrode of FIG. 2.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows a schematic depiction of a section of a gas diffusion electrode 1 produced by means of an extraction process. To this end, a suspension comprising metallic particles 3 and a first polymeric binder material 5 is applied to a metallic support 7 (merely indicated by an arrow). As a result of phase inversion (dipping of the coated metallic support 7 into a “non” solvent), the first binder material 5 solidifies and forms a gas diffusion layer 9 on the surface 11 of the catalyst layer 13 of the gas diffusion electrode 1.

The reaction between a gaseous reactant 15, in the present case CO₂, and the electrolyte 17 then takes place at the metallic particles 3 in this gas diffusion layer 9. This is in the present case a 3-phase reaction in which a conversion of CO₂ into CO occurs at the phase boundary 19 between the metallic particles 3 in the gas diffusion layer 9, the CO₂ and the electrolyte 17 (see also FIG. 4).

In a likewise schematic depiction, FIG. 2 shows a section of a gas diffusion electrode 21 produced by means of a calendering process. Here, the reaction of the CO₂ additionally takes place in the electrolyte 17, which leads to undesirable secondary reactions. Owing to the production process, the gas diffusion electrode 21 has larger pores in the surface, so that there is a risk of undesirable flooding of the gas diffusion electrode 21.

FIGS. 3 and 4 each show corresponding sections 25, 27 of the 2-phase reactions (section 25) and the 3-phase reactions (section 27) as per FIG. 2. FIG. 3 shows a 2-phase reaction. This takes place within the electrolyte 17 and leads, as indicated above, to undesirable secondary products. FIG. 3 shows a 3-phase reaction in which a reaction of CO₂ occurs within the gas diffusion layer 9 of the gas diffusion electrode 1.

The advantages and particular embodiments described for the gas diffusion electrode of the invention and the process of the invention apply equally to the electrolysis cell of the invention and can accordingly be carried over analogously to this.

LIST OF REFERENCE NUMERALS

-   -   1 Gas diffusion electrode     -   3 Metallic particles     -   5 Polymeric binder material     -   7 Metallic support     -   9 Outer layer     -   11 Catalyst surface     -   13 Catalyst layer     -   15 Reactant     -   17 Electrolyte     -   18 Outer layer     -   19 Phase boundary     -   21 Gas diffusion electrode     -   25 Section of gas diffusion electrode     -   27 Section of gas diffusion electrode 

1. A gas diffusion electrode for the utilization of carbon dioxide, comprising: a metallic support, and an electrically conductive catalyst layer which has been applied to the metallic support and has hydrophilic pores and/or channels and hydrophobic pores and/or channels, wherein the catalyst layer comprises metallic particles and a first polymeric binder material and a porous gas diffusion layer containing the first polymeric binder material has been formed on the surface of the catalyst layer.
 2. The gas diffusion electrode as claimed in claim 1, wherein the gas diffusion layer has a porosity of more than 70%.
 3. The gas diffusion electrode as claimed in claim 1, wherein a fluoropolymer is used as first polymeric binder material.
 4. The gas diffusion electrode as claimed claim 1, wherein the thickness of the catalyst layer is in the range from 5 nm to 500 nm.
 5. The gas diffusion electrode as claimed in claim 1, wherein the first polymeric binder material is embedded partly within the pores and/or channels of the catalyst layer.
 6. The gas diffusion electrode as claimed in claim 1, wherein the differential pressure based on the passage of a fluid medium through the gas diffusion layer is in the range from 20 mbar to 220 mbar.
 7. The gas diffusion electrode as claimed in claim 1, wherein the hydrostatic pressure based on passage of a fluid medium through the gas diffusion layer is in the range from 20 mbar to 1000 mbar.
 8. The gas diffusion electrode as claimed in claim 1, wherein the metallic particles are precoated at least in subregions with a second polymeric binder material.
 9. A process for producing a gas diffusion electrode for utilization of CO₂, comprising: mixing of metallic particles with a first binder material to form a suspension, applying the suspension to a metallic support, and introducing the metallic support loaded with the suspension into a precipitation bath to form an electrically conductive catalyst layer, wherein a porous gas diffusion layer containing the first polymeric binder material is formed on the surface of the catalyst layer within the precipitation bath.
 10. The process as claimed in claim 9, wherein a mixture of water and isopropanol is used as precipitation bath.
 11. An electrolysis cell comprising: a gas diffusion electrode as claimed in claim
 1. 12. The gas diffusion electrode as claimed in claim 6, wherein the differential pressure based on the passage of a fluid medium through the gas diffusion layer is in the range from 60 mbar to 200 mbar.
 13. The gas diffusion electrode as claimed in claim 7, wherein the hydrostatic pressure based on passage of a fluid medium through the gas diffusion layer is in the range from 200 mbar to 1000 mbar. 